1. Field of the Invention
The present invention relates to a phase shift mask and a method for fabricating the same, and more particularly to a phase shift mask of the Levenson type provided with a groove formed by etching to a predetermined depth a portion of a transparent substrate on which an edge portion of a phase shift film pattern is disposed, thereby capable of preventing a phenomenon of an undesirable photoresist film residue being left on a silicon substrate. The present invention also relates to a method for fabricating such a phase shift mask.
2. Description of the Prior Art
In conventional methods for the formation of a photoresist film, a photo mask of the type including a photo shield film pattern, for example, a chromium pattern formed on a quartz substrate, has been mainly used as a mask for a lithographic process. Where such a photo mask is used to perform an exposure process in the fabrication of a highly integrated semiconductor device, severe light diffraction and interference phenomenons occur at the boundaries of the photo-shield film patterns of the photo mask. This is because light beams penetrating adjacent photoshield patterns of the mask have the same phase. As a result, no definition of photoresist patterns is generated, thereby degrading the resolution of the photoresist patterns.
In order to define patterns having a critical dimension, a phase shift mask has been proposed, which includes a phase shift pattern in addition to the photo-shield pattern.
In such a phase shift mask, when an incident light penetrates a phase shift layer between adjacent photo shield film patterns, the phase angle of the incident light is shifted through an angle of 180.degree. so that the light emerging from the phase shift layer between adjacent photo shield film patterns has a reverse phase. As a result, the light diffraction and interference phenomenons which occur at the boundaries of the photo shield film patterns are minimized. Accordingly, it is possible to obtain an improved resolution of photoresist film patterns.
An example of a conventional phase shift mask of the Levenson-type will now be described in conjunction with FIGS, 1A and 1B.
FIG. 1A is a plan view of the conventional Levenson type phase shift mask. As shown in FIG. 1A, the phase shift mask includes a transparent substrate 10 and a plurality of uniformly spaced photoshield film patterns 1 formed on the transparent substrate 10. The photoshield film patterns 1 are classified into groups comprising a pair of photo shield film patterns 1. The phase shift mask also includes a phase shift film pattern 2 formed on the transparent substrate 10 such that it overlaps with the patterns of each photo photoshield film pattern group.
FIG. 1B is a plan view of positive photoresist film patterns 5 formed on a wafer 20 by use of the phase shift mask shown in FIG. 1A. The photoresist film patterns 5 exhibit a high resolution by virtue of the effect of the phase of light penetrating each phase shift film pattern 2 of the phase shift mask being shifted through an angle of 180.degree.. However, a residue 3 of the photoresist film is left at the edge portions of each phase shift film pattern 2 directly coated on the transparent substrate 10 because light involving an 180.degree. phase shift meets light not involving a 180.degree. phase shift at the edge portions. The residue 3 results in undesirable residues of patterns formed at subsequent steps. As a result, a poor product is formed.